The metabolic syndrome, which is associated with an high risk for diabetes and atherothrombosis, is associated with hemorheologic abnormalities. These abnormalities seem more and more to be explained by its various symptoms than by insulin resistance which represents theoretically the core of the syndrome. In this study we aimed at defining the specific hemorheologic profile of insulin resistance and hyperinsulinemia by separating a sample of 90 subjects into 4 subgroups according to the clinical score "NCEP-ATPIII" which is the best recognized standardized definition of the syndrome. Results show no significant changes of blood rheology across classes of NCEP score despite a borderline rank correlation between RBC aggregability "M1" and the score. Whole blood viscosity was mostly correlated to HDL-cholesterol (r = -0.353, p = 0.007) and triglycerides (r = 0.574, p = 0.0001). Plasma viscosity was correlated with total cholesterol (r = 0.3359, p = 0.02) and with LDL-cholesterol (r = 0.357, p = 0.03). Red blood cell rigidity "Tk" was negatively correlated to HDL-cholesterol (r = -0.430, p = 0.007). Aggregability "M" was correlated to total cholesterol (r = 0.356, p = 0.01) and "M1" to HDL-cholesterol (r = -0.406, p = 0.006). Thus, despite previously described correlations with glucose disposal parameters, the hyperviscosity syndrome of the metabolic syndrome is not proportional to its clinical scoring and is strongly dependent upon the lipid profile.